Hi I am new to this forum but am so excited to see other people who love their ratties like i do....I would take your puppy to the vet just to be sure...sometimes they do shake cuz they are little or cold that is what i thought about mine but he actually has something called intention tremors.  This is when they "intend" to do something and they just shake because a part of their brain misfires...it could be because they are even thinking of doing something but the shaking usually stops when they are resting or sleeping...my dog does it the worst when it is eating time and when he is excited about something.  a major sign that they have this is the have stiff limbs...like my dog doesn't bend his front legs when he walks...This is not very common though so i would try wrapping it up and keeping it warm but if i continues i figure it never hurts to ask your vet!